A BART police officer looks along the BART tracks along Jones Road in Walnut Creek, where a moving BART train struck and killed two people in Walnut Creek, Calif., on Saturday, Oct. 19, 2013.

The operator of a BART train that struck and killed two workers near the Walnut Creek station on Saturday was in training, a National Transportation Safety Board investigator confirmed Monday afternoon.

NTSB investigators spent Monday conducting extensive interviews with four BART employees including the train operator, his supervisor and a BART dispatcher to try to piece together what caused the accident that killed 66-year-old Laurence Daniels and 58-year-old Christopher Sheppard, NTSB lead investigator Jim Southworth said at a news conference.

The train was running without passengers for training and maintenance during the second day of a strike by BART's two largest unions, Southworth said. Sheppard and Daniels were investigating a report of a dip in the tracks on the Pittsburg-Bay Point line, according to BART.

The two were struck shortly before 2 p.m. Saturday about a mile north of the Walnut Creek station.

Daniels, an Oakland resident, was a contractor working with BART. Sheppard lived in Hayward and was a member of American Federation of State, County and Municipal Employees Local 3993. That union, which is not striking, represents about 210 BART employees, mostly middle managers.

Southworth said preliminary information suggests that the train was moving at about 60 or 70 mph and that the operator, who has held other positions with BART and was in training to be certified as a train operator, had the train in automatic mode. Southworth did not elaborate on what positions or prior training the operator had.

There were six employees on the train at the time of the accident -- two were being trained as train operators, one was training them, and three more were doing troubleshooting and maintenance work, Southworth said. None of those employees were hurt.

During the ongoing investigation, expected to last for weeks or months, NTSB investigators will ship recordings related to the accident back to their Washington, D.C., offices. A re-enactment of the collision is scheduled for Tuesday, as are interviews with BART management, Southworth said.

BART officials initially issued a statement on Saturday saying an experienced operator was at the controls of the train but that it was operating in automatic mode.

However, BART assistant general manager Paul Oversier later partially retreated from that statement, saying he did not yet know who was at the controls when the accident happened.